Errors are not indexed with many=True

[sloria]

When passing many=True to PolyField, errors are not keyed by index like they are with Nested.

Here's an example to reproduce the issue:

from marshmallow import Schema, fields, validate, ValidationError
from marshmallow_polyfield import PolyField


class ChildSchema(Schema):
    foo = fields.Str(validate=validate.Length(max=3))


class ParentSchema(Schema):
    children = fields.Nested(ChildSchema, many=True)


class ParentPolySchema(Schema):
    children = PolyField(deserialization_schema_selector=lambda _, __: ChildSchema, many=True)


try:
    ParentSchema().load({"children": [{'foo': 'x'}, {'foo': 'invalid'}, {'foo': 'alsoinvalid'}]})
except ValidationError as error:
    print('Nested:', error.messages)

try:
    ParentPolySchema().load({"children": [{'foo': 'x'}, {'foo': 'invalid'}, {'foo': 'alsoinvalid'}]})
except ValidationError as error:
    print('PolyField:', error.messages)

Output:

Nested: {'children': {1: {'foo': ['Longer than maximum length 3.']}, 2: {'foo': ['Longer than maximum length 3.']}}}
PolyField: {'children': {'foo': ['Longer than maximum length 3.']}}

I would expect both errors dictionaries to be the same.

[sloria]

One workaround is for the deserialization schema selector to return a schema instance with many=True.

class ParentPolySchema(Schema):
    children = PolyField(
        deserialization_schema_selector=lambda _, __: ChildSchema(many=True)
    )

EDIT: this doesn't work (see comment below).

[sloria]

The workaround doesn't really cut it though because without many=True on PolyField, the data argument of the deserialization_schema_selector is the entire input list instead of a single item, so you can't associate different schemas with different data items.

[sloria]

Ah, List(PolyField(...)) gives the correct output:

class ParentListPolySchema(Schema):
    children = fields.List(
        PolyField(deserialization_schema_selector=lambda _, __: ChildSchema())
    )

This seems like more correct API usage anyway. It might be worth removing the many parameter of PolyField like we're planning to do with Nested: marshmallow-code/marshmallow#779